You can read more about what the Mozilla project is and why it's important over here.

The Mozilla Project needs documentation targetted towards new developers who are starting to work on Mozilla. There is a large learning curve for new developers that needs to be addressed. Part of the problem (for both developers and writers) is that the codebase is constantly in flux, and existing documents lose freshness quickly.

Writing developer documentation for the Mozilla project requires that the writer be familiar with the relevant parts of the codebase and newsgroup discussions. It might be useful for interested writers/editors volunteer to maintain FAQs for the newsgroups, since most technical discussion takes place in those forums.

One way programmer/writers can contribute is by writing documented example code. One imperfect example is at http://lxr.mozilla.org/seamonkey/source/xpcom/sample/. Work is currently underway to allow javadoc style comments so that comments in the code can easily be put on the web.